The Isostructural Substitution‐Induced Growth Mechanism of Rutile TiO2 Electron Transport Layer and the Dominant Distribution for Efficient Carbon‐Based Perovskite Solar Cells

Rutile TiO 2 (R‐TiO 2 ) produced by chemical bath deposition (CBD) is widely considered as the desired electron transport layer (ETL) for perovskite solar cells (PSCs). However, the understanding of the growth mechanism of R‐TiO 2 ETL and its general regular pattern affecting power conversion efficiency (PCE) is underappreciated. Herein, the growth mechanism of TiO 2 on fluorine‐doped SnO 2 substrate (FTO) is demonstrated and it is revealed that pure R‐TiO 2 , rather than a rutile/anatase mixed crystal, is formed under an Sn–Ti isostructural substitution effect. The similarity of lattice parameters and phase structure between FTO and R‐TiO 2 can reduce interface misfit and nucleation barrier, thus boosting heterogeneous nucleation and growth of R‐TiO 2 simultaneously. Based on the key growth conditions of the R‐TiO 2 ETL, the dominant distribution of PCE for hole transport layer (HTL)‐free carbon‐based planar perovskite solar cells is illustrated and discussed, and a champion efficiency of 14.0% is achieved.